Some vehicles such as an automobile have a lock-up clutch in a torque convertor interposed between an engine and an automatic transmission mechanism. As a control of this lock-up clutch, Patent Document 1 discloses a technique in which, in a case where acceleration ON is made at a time of lock-up during a coasting travel (at a time of so-called coasting lock-up), in order to avoid a large torque shock caused by action of a counter torque whose torque transmission direction is reversed, full engagement of the lock-up clutch is forbidden.
Here, after a fully engaged state (i.e. the lock-up) of the lock-up clutch is forbidden by the acceleration ON during the coasting lock-up as described above and the lock-up clutch is in a disengaged state or a slip state, a control that brings the lock-up clutch to the fully engaged state (a lock-up state) again is favorable to improvement in fuel efficiency.
However, in a case where, during the coasting lock-up, an accelerator pedal is depressed only for a short time and subsequently the depression is released, since acceleration OFF is made in a state in which the lock-up clutch is controlled toward the full engagement by the acceleration ON, i.e. during increase in hydraulic pressure supplied to the lock-up clutch, the following problems arise.
For instance, even if the acceleration OFF is made from the acceleration ON, when the hydraulic pressure supplied to the lock-up clutch continues being increased to attempt to bring the lock-up clutch to the fully engaged state, since torque of the engine is decreased by the acceleration OFF, a torque transmission capacity required for the full engagement of the lock-up clutch is decreased, meanwhile, due to the increase of the supplied hydraulic pressure, a torque transmission capacity of the lock-up clutch is increased. Because of this, a balance between an output torque of the engine and the torque transmission capacity of the lock-up clutch suddenly changes, then there is a possibility that the lock-up clutch will suddenly be engaged and a shock will occur.
On the other hand, if the hydraulic pressure having been increased to attempt to bring the lock-up clutch to the fully engaged state is decreased too much by the acceleration OFF, the torque transmission capacity of the lock-up clutch decreases too much, and the lock-up clutch might be in a fully disengaged state. If an acceleration is requested immediately after this full engagement, engine racing (RPM surge) occurs, and fuel efficiency is deteriorated.